Heading into the 2025 season, the Arizona Diamondbacks were thought to have one of the deepest and most accomplished starting pitching groups. In fact they were universally thought to have a top 10 rotation in the game.
Arizona stunned the baseball community by signing Corbin Burnes to a six year, $210 million dollar contract. Along with veterans Zac Gallen, Merrill Kelly, Eduardo Rodriguez, plus the emerging Brandon Pfaadt it was thought this would be a deep unit. Beyond that top five, arguably their best starter in the second half of 2024, Ryne Nelson, was relegated to a long relief role.
Most major outlets in the industry had the D-backs ranked between fourth and sixth in rotation strength. For example, The Athletic and USA today both ranked the D-backs rotation fourth, while Fangraphs and CBS Sports each had Arizona's rotation ranked sixth.
Through 29 games the Diamondbacks rotation has failed to live up to those expectations and projections. The starting pitcher's ERA is 4.56 compared to a league average 4.10, and currently ranks 24th in MLB.
29 games is 18% of the season. Each of the five starters has made six starts, except for Burnes, who will make his sixth start Wednesday night in New York. So each pitcher is roughly one fifth of the way through a full season workload of approximately 30 starts.
From an analytical standpoint, we are in the awkward spot where this is still a somewhat small sample size. A few windblown home runs or poor sequencing results, (i.e. hits coming at the the wrong time) can heavily influence a pure results statistic like ERA.
Therefore at this juncture it's critical that we look at the run estimators and peripheral metrics to better assess underlying performance, and more importantly, get a sense of what to expect going forward. We have several tools to do so including xERA, FIP, and xFIP as explained below. (For those already familiar with these measures, you can skip to the table below)
xERA is the expected ERA the pitcher should have based on a number of factors. The MLB glossary describes it as follows
"xERA takes into account the amount of contact (strikeouts, walks, hit by pitch) and the quality of that contact (exit velocity and launch angle), in an attempt to credit the pitcher or hitter for the moment of contact, not for what might happen to that contact thanks to other factors like ballpark, weather, or defense."
FIP, or Fielding Independent Pitching, strips out everything but walks, strikeouts, and home runs, the events thought to be most under a pitchers control according, to research done over 25 years ago by Voros McCracken. The original incarnation of this metric was DIPS, and the formula for conversion to an ERA scaled version was tweaked by Tom Tango for use at Fangraphs.
xFIP is similar to FIP, with the difference being that homeruns are normalized to a league average home run per fly ball rate.
What do the Run Estimators say about the Diamondbacks Rotation?
There is debate in the analytics community over which estimator best correlates to and predicts future ERA. Various studies have produced differing results. The fact is however that the differences aren't actually that large. To account for that, and level the playing field, I've simply taken an average of the three estimators, (Avg Est. in table above) and compared that to ERA. Note that Baseball Savant does not provide a league average xERA figure.
What we can see here is that the Diamondbacks rotation as a whole has likely performed somewhat better than their ERA results, as the Avg. Est. is .34 runs lower than the actual ERA, coming in at 4.22. That's still worse than the league average ERA of 4.10 however.
Therefore, the best we can say is that the Diamondbacks rotation has performed no better than middle of the pack, even when accounting for "bad luck" or random variance in a small sample.
On an individual basis it's quite stunning to see Eduardo Rodriguez still come in far and away as having the best "under the hood" performance, especially after Tuesday night's drubbing at the hands of the New York Mets. That outing, in which he allowed three homers and gave up eight runs inflated his ERA to 6.06
Brandon Pfaadt meanwhile, has seen somewhat of a reversal of fortune. In the past his run estimators were much lower than his actual ERA. He came into 2025 with a career 5.06 ERA but a 4.15 FIP and 3.88 xFIP. The opposite has been the case this year however, as his ERA is much lower than the estimators predict.
Zac Gallen meanwhile has run into some bad luck with balls in play it appears, as his estimator is also well below his inflated ERA. Unfortunately even that Avg Est. figure of 4.26 is worse than league average.
Rounding out the top five, both Corbin Burnes and Merrill Kelly have somewhat higher run estimates than their already inflated ERA.
Diving Deeper Under the Hood into Peripheral Metrics
To wrap this up we'll take a look into the peripheral metrics that influence both the results and the estimators. At the team level the D-backs are allowing more home runs than league average, with a significantly higher home run per fly ball rate. As a result, the left on base percentage is lower than league average by a large margin as well. See comments below the table for individual takeaways.
Rodriguez has an excellent strikeout to walk ratio, but has an inflated BABIP (Batting average on balls in play) and a homerun per fly balls rate. All of those hits and homers have led to a low Left On Base percentage and an inflated ERA. The estimators indicate that regression to the mean should help him, but that's only going to happen if his confidence isn't shaken and he keeps challengng hitters.
Pfaadt has been intentionally pitching to more contact, trying to get deep into games, and so far that's working. His strikeout rate is down, but his walk rate is minuscule. It will be very interesting to watch how all this plays out and whether he can maintain a sub three ERA with the lower strikeout rate.
Zac Gallen is simply falling behind in counts too often and walking too many batters, especially early in games. This is obvious from the eyeball test. His ground ball rate is very low, and he's allowing a lot of fly balls. So even though his HR/FB rate is about average, he's given up a couple extra homers than normal.
Corbin Burnes has struggled to regain his dominant, swing and miss form, as evidenced by his low strikeout rate. This has actually been a multi year trend, and at this point at least a moderate level of concern is warranted. There were glimpses of improvement in the first four innings of his most recent outing. But until he corrects the swing and miss issue on a consistent basis, he will continue to underwhelm expectations.
Merrill Kelly has uncharacteriscally walked a lot of batters, but with the exception of one disastrous outing in New York, has consistently been able to overcome that and pitch into the sixth inning. The general sense from watching is that he's pitched better than his ERA, although the peripherals and estimators indicate otherwise. Sample size is still an issue here.
Summary
The Arizona Diamondbacks vaunted rotation has seriously underperformed in terms of results. As a group, they cannot claim "bad luck" , as even their peripherals and run estimators indicate a slightly below average performance year to date.
Four of the five starters have a long track record of success, so there is hope they will improve and turn things around. There is no time like the present however. Arizona is on game two of a 16 game in 16 days gauntlet against someof the stiffest competition they'll face all year. Failure to step up now will have repercussions for the team's playoff hopes.
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